The additives we used to use, other than ATF or common remedies, were Redline and Lubro Moly.
On the Mutli port injected cas, the Lubro Moly injector cleaner was superbly effective.
Porsche 911 Engines especially took well to the stuff. They have a Diesel Product, called HiTest, and it magically makes nailing noises from cold Diesels disappear. The stuff is medium priced, but well worth it. Most of the additives at Auto Parts stores are utterly useless. I don't think additives can ever replace frequent maintenance, but they sure can enhance it, if used the right way.
I remember the name of the stuff I put in my Dodge, in a flight of pre-trip shopping frenzy: Z-Max.
this actually made a difference. I never noticed any improvemens from the Slick 50 samples left with us when I had the shop. The Zmax did a good job for the trip, but it was expensive. It got me about 2mpg better in a big Dodge Van, from 12 to 14.5mpg fully loaded, at regular highway speed of about 70-75mph on a trip to Nashville from Los Angeles. Did I say that already?